1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which constitutes a memory cell using a MTJ (Magnetic Tunnel Junction) element which stores “1”- and “0”-information using a tunneling magnetoresistive effect.
2. Description of the Related Art
In recent years, many memories which store information by new principles have been proposed. One of them is a memory using the tunneling magneto-resistive (to be referred to as TMR hereinafter) effect proposed by Roy Scheuerlein et al. (e.g., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest, p. 128).
A magnetic random access memory stores “1”- and “0”-information using MTJ elements. A MTJ element has a structure in which an insulating layer (tunneling barrier) is sandwiched between two magnetic layers (ferromagnetic layers), as shown in FIG. 41. Information to be stored in the MTJ element is determined on the basis of whether the magnetizing directions of the two magnetic layers are parallel or antiparallel.
As shown in FIG. 42, “parallel” means that the two magnetic layers have the same magnetizing direction. “Antiparallel” means that the two magnetic layers have opposite magnetizing directions (the arrows indicate the magnetizing directions).
Normally, an antiferromagnetic layer is arranged on the side of one of the two magnetic layers. The antiferromagnetic layer serves as a member which fixes the magnetizing direction of one magnetic layer and changes only the magnetizing direction of the other magnetic layer, thereby easily rewriting information.
The magnetic layer whose magnetizing direction is fixed is called a fixed layer or pinning layer. The magnetic layer whose magnetizing direction can freely be changed is called a free layer or storing layer.
As shown in FIG. 42, when the magnetizing directions of the two magnetic layers are parallel, the tunneling resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers is minimized. This state is a “1”-state. When the magnetizing directions of the two magnetic layers are antiparallel, the tunneling resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers of the MTJ element is maximized. This state is a “0”-state.
The write operation principle for a MTJ element will be briefly described next with reference to FIG. 43.
MTJ elements are arranged at the intersections between write word lines and write bit lines which cross each other. A write is done by supplying a current to each of a write word line and a write bit line and setting the magnetizing direction of a MTJ element in the parallel or antiparallel state using a magnetic field generated by the currents flowing through the two lines.
For example, assume that the easy-axis (axis of easy magnetization or easy magnetization axis) of an MTJ element corresponds to the X-direction, a write word line runs in the X-direction, and a write bit line runs in the Y-direction perpendicular to the X-direction. In the write mode, a current that flows in one direction is supplied to the write word line, and a current that flows in one or the other direction is supplied to the write bit line in accordance with write data.
When a current that flows in one direction is supplied to the write bit line, the magnetizing direction of the MTJ element is set in the parallel state (“1”-state). On the other hand, when a current that flows in the other direction is supplied to the write bit line, the magnetizing direction of the MTJ element is set in the antiparallel state (“0”-state).
The magnetizing direction of the MTJ element changes in accordance with the following mechanism.
As is indicated by the TMR curve in FIG. 44, when a magnetic field Hy is applied in the longitudinal (easy-axis) direction of a MTJ element, the resistance value of the MTJ element changes by, e.g., about 17%. The change ratio, i.e., the ratio of the resistance difference between the anti-parallel state and the parallel state and the resistance of the parallel state is called “MR ratio”.
Note that the MR ratio changes depending on the structure, composition and morphology of the MTJ element. Currently, even a MTJ element with an MR ratio of about 50% is available.
The synthesized magnetic field of the magnetic field Hy in the easy-axis direction and a magnetic field Hx in the hard-axis (axis of hard magnetization or hard magnetization axis) direction is applied to the MTJ element. As indicated by the solid line in FIG. 45, the intensity of the magnetic field Hy in the easy-axis direction, which is necessary for changing the resistance value of the MTJ element, changes depending on the intensity of the magnetic field Hx in the hard-axis direction. When this phenomenon is used, data can be written in only a MTJ element that is present at the intersection between a selected write word line and a selected data line in memory cells arranged in an array.
This mechanism will be described in more detail using the asteroid curve shown in FIG. 45.
A MTJ element has an asteroid curve indicated by, e.g., the solid line in FIG. 45. The direction of the magnetic field Hx in the write mode is constant. Write data is determined by the direction of the magnetic field Hy.
For example, when the free layer of the MTJ element is magnetized downward in FIG. 45, and a point that indicates the intensity of the synthesized magnetic field of the magnetic field (upward) Hy in the easy-axis direction and the magnetic field Hx in the hard-axis direction is present outside (e.g., at the position indicated by the filled circle) the asteroid curve (solid line), the magnetizing direction of the free layer of the MTJ element can be reversed (downward→upward).
Conversely, for example, when the free layer of the MTJ element is magnetized upward in FIG. 45, and a point that indicates the intensity of the synthesized magnetic field of the magnetic field (downward) Hy in the easy-axis direction and the magnetic field Hx in the hard-axis direction is present inside (e.g., at the position indicated by the open circle) the asteroid curve (solid line), the magnetizing direction of the free layer of the MTJ element cannot be reversed.
In other words, when the intensity of the magnetic field Hy in the easy-axis direction and that of the magnetic field Hx in the hard-axis direction are changed to change the position of the intensity of the synthesized magnetic field in the Hx−Hy plane, the data write for the MTJ element can be controlled.
A read can easily be performed by supplying a current to a selected MTJ element and detecting the resistance value of the MTJ element.
For example, switch elements are connected in series to the MTJ elements. Only the switch element connected to a selected read word line is turned on to form a current path. Consequently, a current flows to only the selected MTJ element. Hence, data of the MTJ element can be read.
In the magnetic random access memory, as described above, the data write is executed by, e.g., supplying write currents to a write word line and a write bit line and causing a thus generated synthesized magnetic field to act on a MTJ element.
In the write operation, it is necessary to always accurately write data in a MTJ element. That is, stable write characteristics are necessary. Stabilizing the write characteristics is especially important when data (the state of a MTJ element) stored in a MTJ element and write data are different. In such a case, the magnetized state (magnetizing direction) of the storing layer of the MTJ element must be stably inverted.
As shown in FIG. 45, when the asteroid curve has a symmetrical shape with respect to the X- and Y-axes, the magnetizing direction of the free layer (storing layer) of the MTJ element can be reversed by a predetermined synthesized magnetic field necessary for magnetization inversion independently of the reversing direction (upward or downward).
However, it is impossible in reality to cause, e.g., all chips (formed from a single or different wafers) manufactured, all memory cell arrays (blocks) in one chip, or all MTJ elements in a memory cell array to have the same MTJ element asteroid curve, i.e., a symmetrical shape with respect to the X- and Y-axes.
In fact, chips, memory cell arrays, word lines/bit lines, or MTJ elements have different MTJ element asteroid curves (asymmetrical shapes with respect to the X- and Y-axes), as shown in, e.g., FIGS. 46 to 49.
In this case, if it is assumed that the synthesized magnetic field Hx+Hy to be used for magnetization inversion has a predetermined intensity, the intensity of the synthesized magnetic field Hx+Hy may sometimes not be able to reach outside the asteroid curve depending on the magnetization reversing direction, and the magnetizing direction of the MTJ element cannot be reversed.
The asteroid curve of the MTJ element becomes asymmetrical with respect to the X- and Y-axes because of various variations in the manufacturing process. Detailed examples are as follows.
{circle around (1)} Shape of TMR Element
Although all MTJ elements are designed to have the same shape, the MTJ elements have actually a subtle shape difference due to manufacturing variations.
The shape of a MTJ element determines the magnitude of a magnetic domain or the intensity of an antimagnetic field (a magnetic field which is generated in a magnetic material and has a direction reverse to the external magnetic field). For this reason, that the MTJ elements have different shapes means that they have different magnetic domain magnitudes or antimagnetic field intensities. That is, the magnetic field intensity necessary for reversing the magnetizing direction of the MTJ element changes between the MTJ elements. Hence, the asteroid curve of the MTJ element becomes asymmetrical with respect to the X- and Y-axes.
{circle around (2)} Thickness/Composition of Magnetic Layer of TMR Element
When the thickness of the magnetic layer (free layer and fixed layer) of a MTJ element increases, the magnetic field intensity necessary for reversing the magnetizing direction also increases. That is, a variation in magnetic layer thickness between MTJ elements makes the asteroid curve of a MTJ element asymmetrical with respect to the X- and Y-axes.
As a magnetic material for the free layer (storing layer) of a MTJ element, an alloy made of iron group elements (Fe, Ni, Co, and the like) is generally used. However, an alloy can have a variation in composition.
If an alloy that forms the free layer of each MTJ element has a variation in composition, saturation magnetization changes between the MTJ elements. In addition, an alloy that forms the free layer of each MTJ element generally has a polycrystalline structure. However, if the magnetic anisotropy of the crystal axis increases, it becomes very difficult to make the asteroid curves of all MTJ elements symmetrical with respect to the X- and Y-axes.
Even if the asteroid curves of all MTJ elements are same and symmetrical with respect to the X- and Y-axes, the write operation may be impossible when the positional relationship between a write line and a MTJ element shifts. That is, even when a necessary synthesized magnetic field is applied to the MTJ element, the magnetizing direction of the free layer of the MTJ element may not reverse.
More specifically, even if the minimum value of a write current that generates a magnetic field necessary for reversing the magnetizing direction is obtained at the time of design on the basis of the ideal MTJ element shape and the ideal positional relationship between the write line and the MTJ element, the write operation may be impossible when the positional relationship between the write line and the MTJ element shifts due to mask misalignment at the time of manufacturing.
As described above, in the conventional magnetic random access memory, some MTJ elements have asteroid curves asymmetrical with respect to the X- and Y-axes because of a variation in shape between the MTJ elements or a variation in thickness/composition between the MTJ elements at the time of manufacturing. Even when the asteroid curve is symmetrical with respect to the X- and Y-axes, the write operation may be impossible when the positional relationship between the write line and the MTJ element shifts. Such phenomena often occur at the early stage of development of magnetic random access memories.
It is therefore necessary to develop an inexpensive reliable magnetic random access memory with a high yield, which can eliminate the phenomenon of disabled write operation due to the variation in write characteristics between MTJ elements for each chip, each memory cell array, each word line/bit line, or each MTJ element by controlling the magnitude of the write current (the intensity of the write magnetic field).